DE 1163784 and DE 3211431, DEGUSSA AG, describes processes for hydrophobicizing silicon dioxide at elevated temperature. For this purpose, the silicon dioxide is activated and predried at high temperatures, giving a hydrophobic product.
DE 4240741, Wacker-Chemie GmbH, describes a process for silylating silicon dioxide with chlorosilanes with addition of lower alkyl alcohols. A uniformly hydrophobic product is obtained.
DE 2344388, Wacker-Chemie GmbH, describes a process for hydrophobicizing silicon dioxide with mechanical compaction. A completely hydrophobic material is obtained.
DE 2403783, BAYER AG, describes a process for preparing a silicon dioxide filler for use in silicone compositions. A completely hydrophobic silica is obtained.
DE 3839900 and EP 0378785, Wacker-Chemie GmbH, describe a process for hydrophobicizing a particulate solid containing Si--OH groups by means of hydrophobicizing agents. This gives a hydrophobic filler which is suitable for use in curable compositions based on diorganopolysiloxanes.
The prior art products are, in general, homogeneously hydrophobic, and are not well suited for some applications. Partially or fully silylated polysilicic acid chains on a silicon dioxide surface have not been previously prepared.